1. Field of the Invention
The present invention relates to a welding joint for fuel tanks, which is welded to a tank wall of a fuel tank to provide a connecting passage between the interior of the fuel tank and the outside.
2. Description of the Related Art
A example of a conventional fuel cut-off valve as a fuel tank welding joint is disclosed in JP 2000-8981A. FIG. 13 is a cross sectional view of a fuel cut-off valve 100. In FIG. 13, the fuel cut-off valve 100 comprises a case main body 102 welded to a tank upper wall FTa, a float 112, a spring 114, and a bottom plate 116. A connecting passage 102a is opened and closed by the float 112 according to a level of the fuel in the fuel tank FT. The upper part of the case main body 102 serves as a lid 104, which is welded to the tank top wall FTa at an annular welded edge portion 106 of the lid 104. When the fuel tank FT is fueled, causing the fuel level to rise, fuel vapor in the fuel tank FT flows out of a float chamber through the connecting passage 102a to the outside. When the fuel reaches a predetermined level FL1 in the fuel tank FT, the flotation force of the float 112 in the fuel cut-off valve 100 increases, causing the float 102 to rise and close the connecting passage 102a, so as to prevent the fuel from flowing out of the fuel tank FT.
The tank upper wall FTa is made of polyethylene, and the lid 104 is made of nylon or a polyacetal. The annular welded edge portion 106 is made of a modified olefinic resin containing a polar functional group, and is thus thermally welded at either end to the lid 104 formed of nylon and the tank upper wall FTa formed of polyethylene. Specifically, polyethylene and nylon are not miscible with each other, but they can both be welded by interposing the modified olefinic resin containing a polar functional group.
In the conventional technology, however, the lid 104 is made of polyethylene, and the fuel vapor in the fuel tank FT can thus penetrate through the lid 104, however slightly, to be released into the atmosphere. Such fuel vapor leaks should be minimized in the interests of environmental safety.